With reference to a product being produced by a continuous reaction, the expression “instantaneous” value of a property of the product herein denotes the value of the property of the most recently produced quantity of the product. The most recently produced quantity typically undergoes mixing with previously produced quantities of the product before a mixture of the recently and previously produced product exits the reactor. In contrast, with reference to a product being produced by a continuous reaction, “average” (or “bed average”) value (at a time “T”) of a property herein denotes the value of the property of the product that exits the reactor at time T.
Throughout this disclosure, the abbreviation “MI” denotes melt index and the abbreviation “FI” denotes flow index.
Throughout this disclosure, including in the claims, the phrase “off-grade product” (e.g., “off-grade” polymer resin) assumes that the product is produced in a reactor with the intention that it meet a specification set (a set of one or more specifications for one or more properties of the product) and denotes that the product has at least one property that does not meet at least one specification in the specification set. For example, if the specification set requires that the product have a resin flow property (e.g., MI) within a specified first range and a density within a specified density range, the product is an off-grade product if its resin flow property (e.g., melt index) is outside the first range and/or its density is outside the density range.
Throughout this disclosure, including in the claims, the phrase “time constant” for change of a product property during a transition is used in a broad sense to denote one or more parameters that determine how rapidly the property changes during the transition, or determine the product's trajectory (value as a function of time) during the transition. Although a property is described herein as changing with a “time constant” during the transition, this does not necessarily imply that the property changes exponentially (e.g., as K e−t/T, where K is an initial value at the start of the transition constant, the time parameter t=0 at the start of the transition, and T is a time constant), although in some embodiments of the invention one or more product properties may change exponentially during the transition. Although a property is described herein as changing with a “time constant” during the transition, this does not necessarily imply that the property changes at a rate that is either fixed or time varying.
One commonly used method for producing polymers is gas phase polymerization. A conventional gas phase fluidized bed reactor, during operation to produce polyolefins by polymerization, contains a fluidized dense-phase bed including a mixture of reaction gas, polymer (resin) particles, catalyst, and catalyst modifiers. Typically, any of several process control variables can be controlled to cause the reaction product to have desired characteristics.
A change from production of one grade of polymer to another typically requires a transition period for a polymerization reactor to switch over to new resin specifications and corresponding process conditions such as reaction temperature, reactants and reactant ratios. During a transition from production of one resin product to another, off-grade polymer material is produced that does not have the desired resin flow characteristic (e.g., melt index), density, or other property of either the initial product or the desired target product. In addition, a polymerization reaction operating under “steady state” conditions can encounter variations that can result in the production of off-grade polymer material that can lead to loss of revenue and reactor shutdown. Since production of off-grade polymer material presents an economic loss, it is desirable to minimize the length of time a reactor produces such material and the amount of material that is produced.
A number of methods have been described to reduce transient, off-grade polymer material. Such methods have involved feeding a polymerization retarder or catalyst poison (e.g., CO2 or O2) into the reactor, adjusting automatic flow ratio controllers to a new value, removing reactant gases from the reactor, reducing the catalyst level, adjusting the amount of the fluidized bed, and/or adding a nonreactive gas such as nitrogen, among other remedial actions.
Despite existing approaches to limit off-grade material, there is a continuing need and desire to provide a more effective and efficient process to reduce the amount of off-grade polymer material produced during the transition to a new product or as a result of a fluctuation during steady state manufacture.
U.S. Pat. No. 5,627,242, issued May 6, 1997, discloses methods for controlling a gas phase fluidized bed polymerization reaction to implement transitions from an initial reaction (in which the product is produced to meet a first set of specifications) to a target reaction (in which the product is produced to meet a second set of specifications). Some such embodiments implement such a transition by changing reaction parameters (e.g., temperature and reactant partial pressure) to predetermined intermediate values and later changing these parameters to their target values (for producing product that meets the second set of specifications) so as to reduce the amount of off-grade material produced during the transition. However, U.S. Pat. No. 5,627,242 does not teach or suggest setting reaction conditions (during performance of a pre-transition reaction in which a product is produced to meet a set of specifications and no off-grade material is produced) before implementing such a transition, to reduce (or minimize) the amount of off-grade material produced during the transition.
U.S. Pat. No. 6,846,884, issued Jan. 25, 2005, discloses methods for controlling resin properties during the production of polyolefins. In some embodiments, the methods implement rapid transitions from an initial reaction (in which the product is produced to meet a first set of specifications) to a target reaction (in which the product is produced to meet a second set of specifications). Some such embodiments employ coordinated manipulation of reaction temperature in combination with a secondary process variable to control resin flow properties to implement the transition (e.g., to move from production of one polymer grade to another) rapidly and in such a manner as to minimize the amount of off-grade material produced during the transition.
U.S. Pat. No. 6,846,884 describes a method for reducing the amount of off-grade polyolefin produced during a transition (during a continuous polymerization reaction) from production of a first polyolefin to production of a second polyolefin, where the second polyolefin is produced with a different reaction temperature than the first polyolefin. The target reaction temperature (for producing the second polyolefin) is compared to the initial reaction temperature, the reaction temperature is then changed (to a value above or below the target temperature as appropriate) and the inflow of the reactant gases modified. At the start of the transition, the reaction temperature is moved down to about 20 degrees C. below the target reaction temperature if the target temperature is lower, or moved up to about 20 degrees C. above the target temperature if higher, and concurrently, the inflow of one or more gases is modified to alter the gas composition in the reactor. The reaction temperature is then moved toward the target temperature as the averaged resin flow value approaches the target value. If desired, the altered reaction temperature and gas composition can be maintained at the initially altered level until the averaged resin flow value of the overall polyolefin in the reactor is within an acceptable range of the target resin flow value of the second polyolefin, whereupon the reaction temperature can be moved toward the target reaction temperature as the averaged resin flow value approaches the target resin flow value.
However, U.S. Pat. No. 6,846,884 does not teach or suggest setting reaction conditions (during performance of a pre-transition reaction in which a product is produced to meet a set of specifications and no off-grade material is produced) before implementing a transition to another reaction, to reduce (or minimize) the amount of off-grade material produced during the transition.